1. Field of the Invention
The present invention relates to amlodipine free base, compositions comprising amlodipine free base and to use of amlodipine free base in therapy.
2. Description of the Related Arts
EP 089 167 and corresponding U.S. Pat. No. 4,572,909 disclose a class of substituted dihydropyridine derivatives as being useful calcium channel blockers. These patents identify that one of the most preferred compounds is 2-[(2-aminoethoxy)methyl]-4-(2-chlorophenyl)-3-ethoxycarbonyl-5-methoxycarbonyl-6-methyl-1,4-dihydropyridine. This compound, which is now commonly known as amlodipine, has the following formula:
While amlodipine as a free base and as a pharmaceutically acceptable acid addition salt are generally taught, the amlodipine examples all make amlodipine maleate; e.g. examples 9, 11, 12, and 22 of EP 089 167. The maleate salt is identified as the most preferred acid addition salt. Surprisingly, amlodipine free base is not characterized. The examples appear to describe the formation of the free base but only as a solution/slurry (example 11) or as a residue remaining after evaporation of the solvent (examples 12 and 22). Only the amlodipine maleate salt is described as precipitating from a solution (examples 12 and 22).
Subsequently, EP 244 944 and corresponding U.S. Pat. No. 4,879,303 were issued directed to the besylate (or benzene sulfonate) salt of amlodipine. The besylate salt is stated to provide certain advantages over the known salts including amlodipine maleate. Various amlodipine salts were compared for water solubility, stability, non-hygroscopicity, and processability for tablet formation. Amlodipine free base was included in the processability testing which involved measuring the amount of amlodipine remaining on the tablet punch after making tablets. The amlodipine free base tablets are reported as leaving on the punch an average of 2.02 μg amlodipine/cm2 per tablet. The amlodipine besylate tablets are reported as leaving on average 1.17 μg amlodipine/cm2 per tablet. Thus, the free base composition suffered from excessive stickiness to the tablet punch and was not as suitable in making solid dosage forms for peroral administration. The amlodipine besylate salt is described in examples 1 and 5 thereof as being made from slurried amlodipine free base although no method is described for how the free base was prepared.
D. M. McDaid and P. B. Deasy in Int. J. of Pharmaceutics 133, 71-83 (1996) suggest the use of amlodipine free base in a transdermal pharmaceutical such as a patch. Amlodipine free base in solid state was prepared in the above article, its structure confirmed by NMR, and it was characterised by a melting point of 144° C. and aqueous solubility of 77.4 mg/l. The solid amlodipine base was prepared by a neutralization of a solution of amlodipine besylate with sodium hydroxide. In a first process, the besylate was dissolved in methanol at 20° C., aqueous sodium hydroxide was added and the base was extracted from the mixture by diethyl ether, which was then evaporated. It should be noted that this process, as described, is irreproducible as diethylether is miscible with the reaction mixture. In a second process, an aqueous solution of the besylate was treated at 50° C. with a sodium hydroxide solution and the base crystallized after cooling to 4° C.
It would be desirable to have an amlodipine oral dosage form based on amlodipine free base. Such a dosage form should preferably be equivalent to amlodipine acid addition salt forms, especially the commercial amlodipine besylate salt, and should not suffer from significant manufacturing or stability problems. Further, it would be desirable to provide a direct method for forming amlodipine free base that can allow for isolation by, inter alia, filtration.